The eye comprises several structurally and functionally distinct vascular beds, which supply ocular components critical to the maintenance of vision. These include the retinal and choroidal vasculatures, which supply the inner and outer portions of the retina, respectively, and the limbal vasculature located at the periphery of the cornea. Injuries and diseases that impair the normal structure or function of these vascular beds are among the leading causes of visual impairment and blindness. For example, diabetic retinopathy is the most common disease affecting the retinal vasculature, and is the leading cause of vision loss among the working age population in the United States. Vascularization of the cornea secondary to injury or disease is yet another category of ocular vascular disease that can lead to severe impairment of vision.
“Macular degeneration” is a general medical term that applies to any of several disease syndromes which involve a gradual loss or impairment of eyesight due to cell and tissue degeneration of the yellow macular region in the center of the retina. Macular degeneration is often characterized as one of two types, non-exudative (dry form) or exudative (wet form). Although both types are bilateral and progressive, each type may reflect different pathological processes. The wet form of age-related macular degeneration (AMD) is the most common form of choroidal neovascularization and a leading cause of blindness in the elderly. AMD affects millions of Americans over the age of 60, and is the leading cause of new blindness among the elderly.
Choroidal neovascular membrane (CNVM) is a problem that is related to a wide variety of retinal diseases, but is most commonly linked to age-related macular degeneration. With CNVM, abnormal blood vessels stemming from the choroid (the blood vessel-rich tissue layer just beneath the retina) grow up through the retinal layers. These new vessels are very fragile and break easily, causing blood and fluid to pool within the layers of the retina.
Diabetes (diabetes mellitus) is a metabolic disease caused by the inability of the pancreas to produce insulin or to use the insulin that is produced. The most common types of diabetes are type 1 diabetes (often referred to as Juvenile Onset Diabetes Mellitus) and type 2 diabetes (often referred to as Adult Onset Diabetes Mellitus). Type 1 diabetes results from the body's failure to produce insulin due to loss of insulin producing cells, and presently requires the person to inject insulin. Type 2 diabetes generally results from insulin resistance, a condition in which cells fail to use insulin properly. Type 2 diabetes of the has a component of insulin deficiency as well.
Diabetes is directly responsible for a large number of disease conditions, including conditions or diseases of the eye including diabetic retinopathy (DR) and diabetic macular edema (DME) which are leading causes of vision loss and blindness in most developed countries. The increasing number of individuals with diabetes worldwide suggests that DR and DME will continue to be major contributors to vision loss and associated functional impairment for years to come.
Diabetic retinopathy is a complication of diabetes that results from damage to the blood vessels of the light-sensitive tissue at the back of the eye (retina). At first, diabetic retinopathy may cause no symptoms or only mild vision problems. Eventually, however, diabetic retinopathy can result in blindness. Diabetic retinopathy can develop in anyone who has type 1 diabetes or type 2 diabetes.
At its earliest stage, non-proliferative retinopathy, microaneurysms occur in the retina's tiny blood vessels. As the disease progresses, more of these blood vessels become damaged or blocked and these areas of the retina send signals into the regional tissue to grow new blood vessels for nourishment. This stage is called proliferative retinopathy. The new blood vessels grow along the retina and along the surface of the clear, vitreous gel that fills the inside of the eye.
By themselves, these blood vessels do not cause symptoms or vision loss. However, they have thin, fragile walls and without timely treatment, these new blood vessels can leak blood (whole blood or some constituents thereof) which can result in severe vision loss and even blindness.
Also, fluid can leak into the center of the macula, the part of the eye where sharp, straight-ahead vision occurs. The fluid and the associated protein begin to deposit on or under the macula swell the patient's central vision becomes distorted. This condition is called macular edema. It can occur at any stage of diabetic retinopathy, although it is more likely to occur as the disease progresses. About half of the people with proliferative retinopathy also have macular edema.
Uveitis is a condition in which the uvea becomes inflamed. The eye is shaped much like a tennis ball, hollow on the inside with three different layers of tissue surrounding a central cavity. The outermost is the sclera (white coat of the eye) and the innermost is the retina. The middle layer between the sclera and the retina is called the uvea. The uvea contains many of the blood vessels that nourish the eye. Complications of uveitis include glaucoma, cataracts or new blood vessel formation (neovascularization).
The currently available interventions for exudative (wet form) macular degeneration, diabetic retinopathy, diabetic macular edema, choroidal neovascular membrane and complications from uveitis or trauma, include laser photocoagulation therapy, low dose radiation (teletherapy) and surgical removal of neovascular membranes (vitrectomy). Laser therapy has had limited success and selected choroidal neovascular membranes which initially respond to laser therapy have high disease recurrence rates. There is also a potential loss of vision resulting from laser therapy. Low dose radiation has been applied ineffectively to induce regression of choroidal neovascularization. Recently ranibizumab and pegaptinib which are vascular endothelial growth factor (VEGF) antagonist, have been approved for use in age-related macular degeneration.
Retinal vein occlusion (RVO) is the most common retinal vascular disease after diabetic retinopathy. Depending on the area of retinal venous drainage effectively occluded, it is broadly classified as either central retinal vein occlusion (CRVO), hemispheric retinal vein occlusion (HRVO), or branch retinal vein occlusion (BRVO). It has been observed that each of these has two subtypes. Presentation of RVO in general is with variable painless visual loss with any combination of fundal findings consisting of retinal vascular tortuosity, retinal hemorrhages (blot and flame shaped), cotton wool spots, optic disc swelling and macular edema. In a CRVO, retinal hemorrhages will be found in all four quadrants of the fundus, whilst these are restricted to either the superior or inferior fundal hemisphere in a HRVO. In a BRVO, hemorrhages are largely localized to the area drained by the occluded branch retinal vein. Vision loss occurs secondary to macular edema or ischemia.
There is therefore a long felt and substantial need for methods of treating diseases of the eye which are characterized by vascular instability, vascular leakage, and neovascularization.